In hot forming the working material is heated up to a certain temperature in which plastic deformation of the material is easier to achieve than at room temperature. Then the material is formed using a die, tool, or punch (hot or at room temperature), and a hydraulic or/and mechanical force, or viscous fluid force (hot or at room temperature) to obtain a desired shape and configuration.
Metal articles can be made by hot forming processes that use complementary forming tools in a press under the pressure of a working gas to stretch-form a preheated metal blank, for example an aluminum alloy sheet blank, against forming surfaces on the forming tools. Hot forming processes include, for example, superplastic forming (SPF), forging, warm stamping, plastic blow forming, warm hydroforming, and quick plastic forming (QPF), described in commonly-assigned U.S. Pat. No. 6,253,588.
In hot blow forming, a highly formable sheet, for example an aluminum alloy sheet, is heated, for example at about 500° C., and gripped at peripheral edges between complementary opposing dies. Pressurized air or other fluid is applied against one side of the sheet to stretch it into conformance with the forming surface of one die. The opposing die provides an air chamber on the pressurized side of the aluminum sheet. Both dies may be heated to elevated forming temperatures to maintain the sheet at a predetermined forming temperature for shaping of the sheet. The sheet may first be pressed against one die for pre-shaping, and then blown against the opposing die for finish shaping. Thus, at least one surface of the hot sheet is stretched against and over the forming surface of a die.
In production operations, heated sheet workpieces are repeatedly placed on the press, formed on the heated die(s), and removed. A lubricant may be applied to the workpieces to buffer the repeated sliding, frictional contact. Low adhesion of the lubricant to the workpieces, however, may decrease the quality of the final part by lowering the tribological performance of the material during hot forming and by increasing the number of surface defects on the final part.